Back rest for a chair



FIG. 1 is a front elevational view showing a back rest for a chair which forms my new design;

FIG. 2 is a right side elevational view thereof;

FIG. 3 is a left side elevational view thereof;

FIG. 4 is a top plan view; and,

FIG. 5 is a bottom plan view thereof. 

The ornamental design for a back rest for a chair, as shown and described. 